1. Technical Field
This invention is directed to a polymer-dispersed liquid crystal composition.
2. Art Background
Polymer-dispersed liquid crystals (PDLCs herein) are compositions in which liquid crystal drops are dispersed in a polymer matrix. Liquid crystals are materials which are liquids in the conventional sense of being in a condensed state that is flowable. Liquid crystals are anisotropic molecules which exhibit long-range orientational order. The particular orientation affects the transmission of light through the liquid crystals.
PDLC films can be used in applications such as flat panel displays because the light-scattering properties of PDLC films can be manipulated. One way to manipulate the light-scattering properties of PDLC films is to apply an electric field to the film. The light-scattering power of the film is different (typically smaller) when the liquid crystal in the drops are aligned by the electric field than when the liquid crystal is not so aligned (i.e. in the absence of the electrical field). Typically, a PDLC film is designed to scatter light in the absence of an electric field and be transparent to light when a voltage is applied across the film. In this case, it is desirable to choose the refractive index of the polymer matrix to be close to the ordinary refractive index of the liquid crystal when the voltage is applied, so that the film is transparent when the voltage is applied thereto. The degree of contrast between the transparent and scattering states of the PDLC film is one parameter that determines the quality of the image, i.e., the higher the contrast, the better the image quality.
There are many ways to make PDLC films. In one scheme, the films are made by combining the liquid crystal composition with a radiation-curable monomer. When the liquid crystal/monomer mixture is exposed to radiation, the monomer polymerizes, thereby forming a polymer matrix. As the monomer polymerizes, the liquid crystal becomes less soluble in the mixture. Due to decreased solubility in the polymer matrix, liquid crystal droplets form during polymerization. The solidification of the matrix prevents the liquid crystal droplets from coalescing, thereby ensuring a stable droplet dispersion throughout the matrix. Typically, the polymer matrix for PDLC films is cured using ultraviolet (UV) radiation. The electro-optical properties of the resulting film depend upon the size distribution of the liquid crystal droplets, their shape, and the interactions between the matrix and the liquid crystal at the droplet surface. Since these parameters are determined, in large part, by the matrix in which the liquid crystal droplets are dispersed, it follows that selection of an appropriate matrix polymer is critical to the properties of the resulting PDLC film.
Monomers that, when polymerized, provide a suitable matrix for liquid crystals for use as PDLC films, are known. Examples of such monomers include acrylates, methacrylates, thiols, and alkyl ethers. For example, PDLC films are formed by combining an alkyl acrylate such as 2-ethyl hexyl acrylate or n-hexyl acrylate with a photoinitiator, a polyfunctional acrylate such as, for example, trimethylol propane triacrylate, and a commercially available liquid crystal mixture, e.g. a liquid crystal mixture such as the one designated TL205 and obtained from E. M. Industries of Hawthorne, N.Y. The film is formed by placing the mixture between glass plates with an electrically conductive coating (e.g. a coating of indium tin oxide) thereon, and exposing the mixture to UV radiation.
One of the difficulties with PDLC films is the high voltage required to switch them from the transparent to the opaque state. For convenience, switching from transparent to opaque is hereinafter referred to as switching from "off" to "on", or vice-versa. Current PDLC films have a thickness of about 10 .mu.m. Although thicker films are advantageous from the standpoint of contrast. between the "on" and "off" states of the film, thicker films also require a higher switching voltage than thinner films. Similarly, it is known that the contrast between the "on" and "off" states of the display is related to droplet size of the liquid crystal in the matrix. However, changing the drop size to improve contrast usually results in a higher switching voltage. Therefore, a PDLC film that provides a greater contrast between "on" and "off" states without a concomitant increase in switching voltage is sought.